FIG. 17 shows a circuit configuration of a high pressure discharge lamp lighting apparatus of a conventional art. In FIG. 17, the high pressure discharge lamp lighting apparatus includes a direct current power source 1, a half bridge circuit 2 consisting of a first switching element 2a and a second switching element 2b for converting direct current voltage of the direct current power source 1 to high frequency voltage, a control circuit 3 for controlling ON/OFF operation of each switching element forming the half bridge circuit 2, a load circuit 4 including a resonant condenser 5, a chalk coil 6, and a starting circuit 7, and a high pressure discharge lamp 8 which is lit by the high frequency voltage supplied from the load circuit 4.
In the high pressure discharge lamp lighting apparatus including the above configuring elements, the operation of each switching component is controlled by the control circuit 3 so as to supply the high frequency voltage having equal to or greater than 1 kHz to the high pressure discharge lamp 8 via a load circuit 4. Further, the control circuit 3 controls the operation of each in order to prevent generation of acoustic resonance phenomena such as “dying out” or “instabilities” accompanied to a bend of a discharge arc inside an arc tube of the high pressure discharge lamp 8, which is well known.
The conventional lighting apparatus for a high pressure discharge lamp employs a configuration which lights the high pressure discharge lamp by setting the lighting frequency to the resonance-free frequency as discussed above. However, it is known to the public that the speed of sound wave within the arc tube changes according to an accumulated lighting time in the high pressure discharge lamp or that the resonance-free frequency band also changes as an electrode exhausts. Conventionally, there has been a problem the high pressure discharge lamp generates the acoustic resonance phenomena such as “dying out” or “instabilities” according to the bend of the discharge arc within the arc tube due to the above various reasons, which prevents the high pressure discharge lamp from keeping steady state lighting.
The present invention aims to solve the above problems and to provide the high pressure discharge lamp lighting apparatus, which always can prevent “dying out” and “instabilities” of the discharge arc within the arc tube even if the resonance-free frequency changes due to the above various factors, and can light the high pressure discharge lamp in a steady state at high frequencies.